Reductive cleavage of the β-O-4 bond in lignin is a rare transformation. One example using a simplified lignin model compound was performed by the Bergman group (Nichols, J. M.; Bishop, L. M.; Bergman, R. G.; Ellman, J. A. “Catalytic C—O Bond Cleavage of 2-Aryloxy-1-arylethanols and its Application to the Depolymerization of Lignin Related Polymers” J. Am. Chem. Soc. 2010, 132, 12554-12555). In this publication, a Ru-based catalyst performed the cleavage to generate the acetophenone and the phenol. A disadvantage is that inert atmosphere was required for efficient catalysis.
Very recently, a Ni catalyzed reduction of different model compounds and also pyrolysis oil was reported using isopropanol as hydrogen donor (X. Wang, R. Rinaldi, “Exploiting H-transfer reactions with RANEY Ni for upgrade of phenolic and aromatic biorefinery feeds under unusual, low-severity conditions”, Energy Environ. Sci., 2012, 5, 8244). The main transformation described in said publication is the reduction of the aromaticity in aromatic compounds to generate the saturated hydrocarbons. The authors also show with a few examples that phenolic and benzylic ethers are cleaved to generate saturated alcohols or hydrocarbons. However, the authors do not include the β-O-4 bond in a simplified or parent model. This bond is much more difficult to cleave than to cleave the highly activated phenolic and benzylic bonds, which are considered standard procedures. Another disadvantage with the previous report using Ni was that an excess of the metal was used. Thereby, the metal was not used in catalytic amount, and may only be considered to mediate and not catalyze the reaction.
It is well known that Ni is active in the hydrogenolysis of aryl ethers using hydrogen gas (A. G. Sergeev, J. F. Hartwig, “Selective, Nickel-catalyzed Hydrogenolysis of Aryl Ethers” Science, 2011, 332, 439-443). Also, that Ni and hydrogen gas or hydrogen donor is active in the reduction of the aromaticity in phenols and other aromatic compounds (C. Zhao, Y. Kou, A. A. Lemonidou, X. Li, J. A. Lercher, “Hydrodeoxygenation of bio-derived phenols to hydrocarbons using RANEY Ni and Nafion/SiO2 catalysts,” Chem. Commun., 2010, 46, 412-414).